The invention relates to a new electrolysis cell, without a diaphragm, particularly for continuous production of alkali metal chlorates and, in particular, sodium chlorate by electrolyzing a liquor containing sodium chloride, although it may equally be applied to the production of alkali hypochlorites or perchlorates
Since the first commercial electrochemical production of chlorates goes back to over a century ago, it is not surprising that many types of cells have been proposed for this purpose. Since cells for chlorates normally have no diaphragm, one might think at first sight that they are simple cells, differing from one another only in a few technological details. This would be to overlook the fact that fairly complex phenomena take place in them, due particularly to the existence of a large number of reactions varying greatly in their kinetics.
Thus, in addition to the main anode and cathode reactions liberating chlorine and hydrogen, there are chemical reactions leading ultimately to the formation of chlorate, as well as parasitic reactions.
Thus, the equation EQU 3H.sub.2 O + NaCl .fwdarw. NaClO.sub.3 + 3 H.sub.2,
which is generally quoted to convey the whole phenomenon, gives an overly simplified view of the phenomena which take place and does not allow for the fact, e.g., that the reaction whereby chlorate is formed from hypochlorous acid is slow whereas the anode and cathode reactions are fast.
This explains why two very different approaches have come to the fore among the technological solutions proposed, one maintaining that the chemical reactions must take place as far as possible outside the cell, and the other, conversely, that everything should take place inside one and the same cell.
The latter concept is particularly attractive since it makes it possible to construct cell component arrangements which are more compact and, a priori, simpler. However, it encounters many difficulties in practice, due to the fact that the electrolytes need to be circulated, mixed and reacted inside the cell, for the reasons just stated, and also due to the fact that these arrangements must satisfy electro-chemical and electrotechnical requirements, such as the passge of the current, or thermal requirements such as dissipation of the heat produced, or kinetic requirements such as the need to being the various reagents together under specific conditions.
The practical problems which arise also include that of evacuating from the cell the gases formed. To facilitate interpolar release of gas, it has already been proposed, in French Pat. No. 947,057, to use cathodes comprising perforated metal plates which may contain 60% cavity. However, accumulation of gas in the interpolar space is known to exclude electrolyte from the space and consequently to increase electrical resistance between anode and cathode, thereby increasing the voltage and reducing the energy yield of the cell.
Attempts have been made to lessen this drawback by eliminating the gas as rapidly as possible from the critical space where the gases are formed. French Pat. No. 2,029,723 thus proposes the use of a cathode comprising a rear plate and a pervious plate, the pervious plate being located between and at a certain distance from the rear plate and the anode and having an oblique surface so as to allow gas to pass into a space provided between the rear plate and the pervious plate.
French Pat. No. 2,156,020 proposes a cell with a section for the formation of chlorates, located in the bottom of the cell below the active section, the active section being provided with deflectors in order to lengthen the reaction whereby hypochlorite is converted into chlorate.
U.S. Pat. No. 3,055,821 proposes a cell for high-temperature production of chlorates, designed so that the electrolyte circulates in a cell due to an ascending force resulting from the hydrogen formed being released between the electrodes and dropping onto the sides of the cell. A cell of this type has three stationary sides and one anode-carrying side, the anodes being arranged between the pairs of cathodes, and isolating spacers being disposed between the anodes and the cathodes.
All these solutions aim to produce the same result, viz., to improve the circulation of electrolyte, and have produced interesting results. But it is known that the present-day profitability requirement is becoming more and more critical, particularly as far as energy consumption is concerned.
Furthermore, in order to obtain good dimensional stability and durability and to increase current density, there is an increasing tendency to use metal anodes with dimensions which remain constant with the passage of time.
Use of these anodes has allowed maximum reduction in the interpolar distance, but the need to circulate the electrolyte and evacuate the gases has become more critical, since these anodes enable the cell to be operated at high temperatures. Finally, such cells must have minimum bulk and be simple enough in design to make them easy to construct, maintain and operate.
The cell which the present invention seeks to provide must, in particular, be simple from the technological point of view, must avoid the inclusion of complex circuits with large external volumes, thus eliminating the dangers of corrosion. It must be capable of operation at high temperature and must avoid the disadvantages which result from previous proposals, such as inclined ortions or additional components such as deflectors, complementary plates, etc.
The present invention also aims to provide a cell which will give maximum benefit from the use of electrodes with constant dimensions. These allow a reduction in interpolar space, thus enabling the operating voltage to be lowered, while at the same time avoiding the main drawback of such an arrangement, viz., the accumulation of gases in the interpolar space.
It is, therefore, an object of the present invention to provide an electrolytic cell, suitable for the production of alkali chlorates, which overcomes the disadvantages of the prior art.
It is also an object of the present invention to provide an electrolytic cell which provides the aforestated advantages of the present invention.